The invention concerns a method of operating a generator connected to a power supply network, in particular a synchronous generator, during a network fault in the power supply network, in particular during an electric short-circuit.
During a network fault in a power supply network, in particular during an electric short-circuit and the concomitant drop in network voltage in the power supply network, unwanted changes in operating parameters of an electric generator connected to the power supply network, in particular a synchronous generator, such as changes in the rotary speed or the load angle of the generator, can occur. The term load angle is used as is known to denote the angle between the vector of the rotating magnetic field in the stator of the generator and the vector of the rotating magnetic field in the rotor of the generator.
The drop in the network voltage leads to a significant reduction in the delivery of electric power from the generator to the power supply network. In the case of usual configurations in which a rotor of the generator is connected to an engine shaft, driving the rotor, of an internal combustion engine (for example a gas engine) that drop in electric power can lead to a corresponding increase in the rotary speed of the internal combustion engine and therewith the rotor. As a result, synchronization of the generator with the power supply network can be lost or damage in the generator can even be caused.
Detection of a network fault in the power supply network can be effected for example by the network voltage of the power supply network and/or the electric current fed into the power supply network by the generator and/or the rotary speed of the generator or the internal combustion engine and/or the torque at the engine shaft of the internal combustion engine or at the rotor shaft of the generator being monitored, in which case upon the occurrence of a change in at least one of those monitored operating parameters beyond a predeterminable threshold value a network fault is detected. In that respect, changes which occur can be detected as a network fault only when a plurality of those operating parameters involve corresponding changes beyond predeterminable threshold values, if therefore for example both the network voltage, the electric current and also the rotary speed involve corresponding deviations. The generator can remain connected to the power supply network during a network fault.
The conventional approach for reacting to such network faults is to take suitable measures in order to counteract such an increase in the rotary speed and an increase related thereto in the load angle of the generator. Thus measures are usually taken, which reduce the rotary speed and the load angle. Such a measure by way of example is a reduction in the acceleration moment by an internal combustion engine connected to the generator being correspondingly throttled.
It has been found, however, that the conventional measures are disadvantageous in the event of a network fault in certain situations. Thus, the rotary speed of the generator may not increase upon the occurrence of a network fault but firstly falls. That effect which is known to the man skilled in the art by the English technical term “back-swing” can under some circumstances even lead to pole slippage of the generator. Pole slippage in turn leads to an instability of the generator, in which a mechanical power introduced into the rotor by an internal combustion engine by way of the engine shaft can no longer be converted as desired into electric power by the generator.